osd/ECOmapJournal: bind by reference when clearing omap keys

The clear_omap loop bound each key_map entry by value, so
update_value() mutated a copy and cached keys were never marked
removed.

As a result, clear_omap had no effect on already-cached keys: cleared
omap keys retained stale values and were returned on read.

Introduced-by: 7289e4d2ece ("osd: Add ECOmapJournal class and relocate OmapUpdateType enum class")
Fixes: https://tracker.ceph.com/issues/77750
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>